The present invention relates generally to method and apparatus for recording an image on a developer sheet having a surface coated with a developer material while using a photosensitive pressure-sensitive recording medium having a surface coated with an immense number of pressure-rupturable microcapsules (hereinafter referred to as "a microcapsule sheet"). More particularly, the invention relates to method and apparatus for performing pressure development with a pair of pressurizing rollers, in which timings at which the rollers are brought to nip and retract positions are determined in precise manner.
In an image recording apparatus of the type using recording media comprising an elongated, web-like microcapsule sheet and a developer sheet in the form of cut sheet, the microcapsule sheet which has been accommodated in a cassette in a rolled state is withdrawn therefrom and is exposed to imaging light to form a latent image thereon. The developer sheet is fed out from another cassette typically at a timing in coincidence with the start of the exposure onto the microcapsule sheet and is conveyed at a speed in synchronism with the conveying speed of the microcapsule sheet so that the developer sheet is superposed on the exposed region of the microcapsule sheet. The two sheets thus superposed are introduced into a developing station comprising a pair of pressurizing rollers to thereby develop the latent image on the microcapsule sheet and reproduce a visible image on the developer sheet while applying a high pressure to the superposed sheets.
The pressurizing rollers are selectively held in spaced apart condition (retract position) and an intimate contact condition (nip position). The pressure development can be performed when the rollers are in the nip position. The rollers have been held in the retract position to allow the sheets to be introduced into a space between the two rollers. When the leading ends of the two sheets are introduced thereinto, the rollers are moved toward each other and held in the retract position at a predetermined timing (hereinafter referred to as "nip timing") to thus start the pressure development. As the pressure development proceeds and at a timing when the development in the exposure zone has terminated, the rollers are brought to the retract position. This timing will herein after referred to as "retract timing". The nip timing is determined based on the detection of the leading end of the developer sheet at a predetermined position upstream of the developing station, and the retract timing is determined based on a predetermined number of rotations of one of the pressurizing rollers counted from the nip timing. The retract timing may alternatively be determined based on the detection of the trailing end of the developer sheet. Specifically, it may be determined based on a predetermined number of rotations of one of the pressurizing rollers counted from the trailing end of the developer sheet.
In the case where the retract timing is determined in association with the detection of the leading end of the developer sheet, the pressurizing rollers are still held in the nip position if the developer sheet used is of short length. In this case, only the microcapsule sheet is applied with the pressure after the developer sheet has been transported past the pressurizing rollers. As a result, the pressurizing rollers are smeared with a chromogenic material released with the ruptured microcapsules. The smeared rollers are incapable of performing uniform pressure development for the sheets which follow. There may be cases where the developer sheet is wrinkles due to the transportation by the smeared rollers.
On the other hand, in the case where the retract timing is determined in association with the detection of the trailing end of the developer sheet, the above-mentioned drawback can be obviated. However, due to the variation in the setting position of the sensor for sensing the trailing end of the developer sheet, the occurrence of the retract timing is not in agreement with the actual trailing portion of the exposed zone on the microcapsule sheet, whereby the trailing end position of the reproduced image becomes different in the respective apparatuses.
Even if the regular size developer sheet is used, it is necessary that the developer sheet be introduced into the developing station without fail. Otherwise, the pressurizing rollers are smeared with the chromogenic material as described above. To ensure the feeding of the developer sheet, it has been proposed to employ a sensor for sensing the leading edge of the developer sheet and to dispose feed rollers at a position upstream of the developing station for conveying the developer sheet a predetermined distance toward the developing station upon sensing the leading edge of the developer sheet. However, if the feed rollers have been smeared or if double-superposed developer sheets have been reached, the feed rollers tend to slip with respect to the developer sheets. As a result, the developer sheet is not fed into the developing station, whereby the microcapsule sheet is solely applied with pressure and thus the smearing of the pressurizing rollers results.
In the recording apparatus provided with an exposure amount adjusting function or a copying magnification selection function, typically the conveying speed of the microcapsule is changed depending upon the exposure amount as set or the selected copying magnification. However, the nip timing is not changed attendant to the change of the conveying speed of the microcapsule sheet, with the result that the nip timing is advanced or delayed. In the case where the nip timing is advanced, i.e., when the pressure application is taken place before the developer sheet has been introduced, the microcapsule sheet is solely applied with the pressure, whereby the smearing of the pressuring rollers results. On the other hand, in the case where the nip timing is delayed, i.e., when the pressure application is taken place after the leading exposure zone has passes over, the leading exposure zone will not be subjected to pressure development. Further, the retract timing is advanced or delayed corresponding to the advancement or delay of the nip timing, since the pressurizing rollers are brought to the retract position when the superposed sheets are conveyed a predetermined distance by the pressurizing rollers, which distance is determined by the number of rotations of one of the pressurizing rollers. To summarize, when the exposure amount is adjusted or when enlarged or reduced size copy is to be obtained, the image location on the developer sheet is displaced and a part of the image is not reproduced.
When the same original document is successively copied, a problem exists such that the trailing end portion of the precedingly fed developer sheet remains in the nip between the pressurizing rollers when the subsequently fed developer sheet is to be introduced into the gap between the pressurizing rollers. This is due to the fact that the timing at which the feeding of the developer sheet is performed in coincidence with the start timing of the exposure. Under such a condition, the subsequently fed developer sheet cannot be introduced into the gap between the pressurizing rollers, or the leading end portion of the subsequently fed developer sheet is overlapped on the trailing end portion of the precedingly fed developer sheet and introduced thereinto in the overlapped condition. In the latter case, the overlapped portions are not properly pressure developed.